Various types of wafer-handling robots are known for transporting the wafers to and from the FOUP and among processing stations. Many such robots employ a robotic arm having a spatula-shaped end that is inserted into the cassette to remove or insert a wafer. The spatula-shaped end of the robotic arm is commonly referred to as an end effector. One type of end effector secures the wafer contact surface to the wafer by a vacuum source. The vacuum source pulls the bottom surface of the wafer into contact with the entire wafer contact surface.
With many workpieces, and certainly with semiconductor wafers, the surfaces of the workpieces can be easily damaged if the wafer-handling robot contacts the top or bottom surface of the wafer. Because of this, the wafer-handling robot should preferably contact only the peripheral edge of the wafer, or at most, the bottom surface within a narrow distance from the edge (known within the semiconductor industry as the “edge exclusion zone”). A wafer-handling robot must load wafers into and unload wafers from a wafer processing apparatus with a high degree of precision to avoid contacting the critical surfaces of the wafer.
FIG. 1 illustrates a conventional wafer-handling end effector. The wafer-handling robot 10 supports a wafer 12 by a wafer blade 11 that has a proximal end 12 and a distal end 14. The distal end 14, in this configuration, has two spaced apart and substantially parallel fingers—a first finger 16 and a second finger 18. The first finger 16 includes a distal wafer support 20 that contacts the wafer 12 either along the peripheral edge or the Exclusion Zone. The second finger 18 also includes a distal wafer support 22 that contacts the wafer in a similar manner.
The wafer blade 11 also includes proximal wafer supports 24 and 26. The proximal wafer supports 24 and 26 also contact the wafer 12 either along the peripheral edge of the wafer 12 or along the Exclusion Zone. The distal wafer supports 20 and 22 include a backstop portion to prevent the wafer 12 from sliding off the wafer blade 11 during transport. A wafer 12 is not always perfectly positioned on the wafer blade 11 such that the peripheral edge of the wafer rests against the wafer supports 20 and 22 at all times. Sudden movement or high rotational speeds by the wafer handling robot 10 may throw the wafer 12 against the supports 20 and 22 and cause damage to the wafer 12, or cause the wafer 12 to slip over the wafer supports 20 and 22 and off the blade 11. An example of an end effector similar to that shown in FIG. 1 is disclosed in U.S. Pat. No. 6,077,026, issued to Schultz, entitled “Programmable Substrate Support For a Substrate Positioning System,” which is assigned to Asyst Technologies, Inc., and is incorporated herein by reference.
As the robot speed and acceleration increase, the amount of time spent handling each wafer and delivering each wafer to its next destination is decreased. The desire for speed, however, must be balanced against the possibility of damaging the wafer or the film formed on the wafer. If a robot moves a wafer too abruptly, or rotates the wafer blade too fast, the wafer may slide off the blade, potentially damaging the wafer. Further, particle contaminants may result if the wafer slides around on the end effector. In addition, movement of the wafer on the wafer blade may cause substantial misalignment of the wafer. A wafer that is not aligned may result in inaccurate processing or even additional particle generation.
Wafer handling robots that grip the wafer by its peripheral edge exist today. By way of example only, one type of robot end effector includes an active contact point that moves between a retracted wafer-loading position and an extended wafer-engaging position that urges the wafer against a pair of distal wafer supports. Another example of a robot end effector includes a pair of arms that grip the peripheral edge of the wafer after the wafer is placed on the blade. The active contact point and the pair of arms retract and extend through a vacuum or pneumatically actuated mechanism. Vacuum and pneumatic actuation mechanisms provide poor control of the maximum force exerted on the wafer, because velocity of the active contact point or the arms cannot be controlled.
After the end effector lifts a wafer off a cassette support, the active contact point (or the pair of arms) extends to contact the peripheral edge of the wafer and push the wafer against the distal wafer supports. The active contact point holds the wafer in place on the wafer blade. A vacuum actuated gripping device, for example, cannot stop or vary its speed between the retracted wafer-loading position and the extended wafer-engaging position.
An end effector that incrementally controls the motion of the gripping device, which prevents damage to the wafer, is a desirable feature. An end effector having a real-time force feedback system that monitors the force exerted on the wafer is also a highly desirable feature.